High-voltage CMOS-compatible capacitors

ABSTRACT

A high-voltage stacked capacitor includes a first capacitor and a second capacitor. Each capacitor includes a first plate having a first semiconductive body and a second plate having a floating electrode. The first and second semiconductive bodies are electrically isolated from each other. The floating electrode includes an intercapacitor node configured to self-adjust to a value less than a working voltage impressed on the stacked capacitor.

RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 10/635,247, filed on Aug. 5, 2003, U.S. Pat. No. 6,842,327entitled “High-Voltage CMOS-Compatible Capacitors” and naming ChrisDiorio and Frederic Bernard as inventors, the disclosure of which ishereby incorporated herein by reference.

TECHNICAL FIELD

This disclosure describes high-voltage capacitors that may be formedusing standard complementary metal-dielectric-semiconductor orcomplementary metal-oxide-semiconductor (CMOS) integrated circuit (IC)processing technology. More particularly, this disclosure describeshigh-voltage CMOS-compatible capacitors that include stackedmetal-dielectric-semiconductor or semiconductor-dielectric-semiconductorfloating-gate structures which have simpler ancillary circuitrequirements than prior art structures because they include aself-biasing intercapacitor node.

BACKGROUND

FIG. 1 is an exemplary schematic diagram of a capacitor 100 inaccordance with the prior art. The capacitor 100 includes a first plate110, a first electrode 115 coupled to the first plate and permittinginterconnection with other circuit elements (not shown in FIG. 1), asecond plate 120 together with a second electrode 125 coupled to thesecond plate 120 and a dielectric 130 disposed between the first 110 andsecond 120 plates of the capacitor 100. The capacitor 100 operates bystoring and releasing charge in response to signals coupled to the first115 and second 125 electrodes. It is generally desirable to formcapacitors 100 together with CMOS circuit elements but without requiringmodification of standard CMOS processes. This allows greater choice offoundry for CMOS IC manufacturing, simplifies production, and reducesexpense in realizing ICs that include the capacitor 100.

Some applications for capacitors 100 require larger breakdown voltagesthan are needed for most of the other circuit elements forming the IC.Such applications may include power supplies associated with programmingcircuitry for programmable or nonvolatile memory elements.

The first 110 and second 120 plates may be realized in a number of formsin ICs using standard CMOS processes. These forms can include metal orsemiconductor layers comprising the capacitor plates 110, 120, separatedby a layer forming the dielectric 130, or interdigitated conductivepatterns comprising the capacitor plates 110, 120.

Capacitors 100 formed using interdigitated conductive patterns for thecapacitor plates 110, 120 tend to provide relatively high breakdownvoltage. These also tend to be relatively large and to providerelatively little capacitance per unit area of the IC in which they areformed.

Capacitors 100 may also be formed by using a first conductive plate 110,which may be relatively planar, formed on or in a substrate, arelatively planar dielectric 130 disposed atop the first plate 110 and arelatively planar second conductive plate 120 formed on and supported bythe dielectric layer 130. The first plate 110 may be formed from metalor from a conductively doped semiconductive layer. When formed on asemiconductive substrate using conventional CMOS processing techniquesand employing a dielectric analogous to a FET gate dielectric, suchcapacitors may be referred to as MOS capacitors or MDS(metal-dielectric-semiconductor) capacitors, although the dielectric mayor may not be an oxide or silicon dioxide, and the first 110 and second120 plates may be semiconductive or other conductive material ratherthan including metal.

For example, first 110 and second 120 plates may be formed usingpolysilicon or metal layers separated by a dielectric layer 130 formedusing polyimide or an oxide such as silicon dioxide. Such capacitors 100may be formed to have relatively high breakdown voltage using arelatively thick dielectric layer 130 but then provide relatively littlecapacitance per unit surface area of the IC because of the thickness ofthe dielectric layer 130.

Alternatively, MDS structures may be formed either as FETs or as MDScapacitors, using the same material for the dielectric 130 as is used toform gate dielectrics for other FET structures that are also part of theIC. This allows the capacitor 100 to be formed via the same processesthat are employed to form transistors and the like. However, gatedielectrics are typically made relatively thin because, among otherthings, the transconductance of a FET per unit gate width (i.e., perunit area of the IC) decreases as the gate dielectric thicknessincreases. As a result, while use of dielectric layers 130 that are alsosuitable for FET gate structures provides relatively high capacitanceper unit area, the breakdown voltage of the resulting capacitor 100 isrelatively low.

Various prior art capacitors have been employed to address theseconflicting aspects of high-voltage capacitor realization. For example,U.S. Pat. No. 5,187,637, entitled “Monolithic High-Voltage Capacitor”and issued to Embree describes a capacitor employing multiple dielectriclayers and a guard ring but requires process steps not normallyassociated with CMOS IC fabrication, while U.S. Pat. No. 5,587,869entitled “High-Voltage Capacitor Manufacturing Method And High-VoltageCapacitor” and issued to Azumi et al. describes a high-voltage capacitornot suitable for monolithic integration.

Various prior art capacitors have been employed to address theconflicting aspects of high-voltage capacitor realization in the contextof CMOS ICs. For example, U.S. Pat. No. 6,137,153, entitled “FloatingGate Capacitor For Use In Voltage Regulators”, issued to Le et al.describes a single floating-gate NMOS transistor used together withother circuitry as a constant-capacitance capacitor in a voltage-dividerstructure for voltage regulation, while U.S. Pat. No. 6,262,469,entitled “Capacitor For Use In A Capacitive Divider That Has AFloating-Gate Transistor As A Corresponding Capacitor” and issued to Leet al. describes one or more floating-gate NMOS transistors usedtogether with other circuitry to provide a capacitive voltage dividerfor voltage regulation. These circuits are not intended to providehigh-voltage capacitors; they also require a conventional control gatedisposed atop the floating gate in order to operate, but the processingrequired to fabricate such control-gate structures is not available inmany modem CMOS foundry processes.

One approach that has been employed to address these conflicting aspectsof high-voltage capacitor realization in CMOS ICs is to stack a pair ofFETs with source/drain contacts connected together, or a pair of MOScapacitors, in series by forming each capacitor structure so that theyshare a contact in the form of a well of semiconductive material. FIG. 2is an exemplary schematic diagram of a high-voltage capacitor 200 formedfrom two MOS capacitor structures 205 and 210, in accordance with theprior art. For example, U.S. Pat. No. 5,926,064, entitled “Floating GateMOS Capacitor” and issued to Hariton describes a voltage-independentcapacitor circuit analogous to that of FIG. 2 and describes theassociated biasing circuitry required for such.

Each of the MOS capacitor structures 205 and 210 of FIG. 2 isillustrated as comprising a PMOS FET-like structure having a respectiveexternally accessible capacitor electrode 212, 214, a floating gate 231and control electrode 232, source/drain electrodes or power electrodes215, a channel 220 electrically coupling the source/drain electrodes 215of each structure 205 or 210, and a body or well contact 225. In each ofthe structures 205 or 210, the gate structure includes a dielectric 230such as a conventional gate oxide, and is capacitively coupled to thesource/drain electrodes 215 and channel 220 through the dielectric 230.

Each FET structure 205, 210 is shown as comprising a floating-gateelectrode 231 formed on a first portion of the dielectric layer 230 anda control-gate electrode, 232, formed on and separated from thefloating-gate electrode 231 by a second portion of the dielectric layer230. Conventional floating-gate transistors incorporate separatefloating gate 231 and control gate 232 structures, with only the controlgate 232 typically being electrically accessible.

A conventional stacked capacitor 200 is formed by interconnecting one ofthe source/drain contacts 215 from each of the FETs at a commonintercapacitor node 235 and employing the control-gate electrodes 212and 214, respectively, as the first 115 and second 125 capacitorelectrodes (FIG. 1). One issue with such stacked capacitors 200 is thatconventional MOS or floating-gate transistor structures have no meansfor adjusting charge on common node 235, and, consequently, are unableto avoid one of the two structures 205, 210 carrying a disproportionateamount of the voltage impressed across the electrodes 212, 214.

Furthermore, the semiconductive structure forming the commonsource/drain region has leakage currents associated with it. As aresult, a bias circuit (not illustrated) is required in order tomaintain the intercapacitor node 235 voltage and to provide a path foraddition or removal of charge caused by leakage currents. Requiring suchbias circuitry for each high-voltage capacitor 200 undesirably increasesthe complexity of the resulting integrated circuit that includes thehigh-voltage capacitors 200 and results in poorer form factor, i.e., alarger circuit layout.

Accordingly, improved techniques and structures are needed for realizinghigh-voltage capacitors having a compact form factor using standard CMOSprocesses.

SUMMARY

An architecture and methodology for provision of high-voltage stackedMOS or MDS capacitors that may be realized using standard CMOS-typeprocessing is described. In one aspect, such a stacked capacitorincludes a first capacitor and a second capacitor. Each capacitorincludes a first plate comprising a first semiconductive body and asecond plate comprising a first floating electrode. The first and secondsemiconductive bodies are electrically isolated from each other. Thefirst and second floating electrodes are connected together at anintercapacitor node configured to self-adjust to about one-half of aworking voltage impressed on the stacked capacitor.

BRIEF DESCRIPTION OF THE CONTENTS

FIG. 1 is an exemplary schematic diagram of a conventional capacitor.

FIG. 2 is an exemplary schematic diagram of a conventional stackedhigh-voltage capacitor formed from two MOS capacitor structures.

FIG. 3 is an exemplary schematic diagram of an embodiment of a stackedhigh-voltage capacitor that is compatible with conventional CMOS-typeprocesses.

FIG. 4 is an exemplary plan view of an embodiment of the stackedhigh-voltage capacitor of FIG. 3.

FIG. 5 is a side view, in section, taken along section lines V—V of FIG.4, of an embodiment of the stacked high-voltage capacitor of FIGS. 3 and4.

FIG. 6 is a side view, in section, analogous to that of FIG. 5, of anembodiment of the stacked high-voltage capacitor of FIGS. 3 and 4.

FIG. 7 is a side view, in section, analogous to that of FIG. 5, of anembodiment of the stacked high-voltage capacitor of FIGS. 3 and 4.

FIGS. 8A and 8B are band diagrams reflecting different bias andtunneling conditions relative to the embodiments of FIGS. 3–7.

FIG. 9 is a schematic diagram of a stacked high-voltage capacitorincorporating a charge injector and a charge drain.

FIG. 10 is a schematic diagram of an exemplary voltage multipliercircuit advantageously employing the capacitor structures of any ofFIGS. 3–7.

DETAILED DESCRIPTION

The following disclosure describes simplified high-voltage stackedcapacitors that may be realized using standard CMOS processes. Theability to form the stacked capacitors using standard CMOS processingtechniques facilitates fabrication and integration of the resultingstacked capacitors together with other circuit elements and allowsbroader choice of foundries for such fabrication. This is particularlyhelpful for realizing CMOS devices having need for some amount ofhigh-voltage circuitry. Enabling use of standard processing reducescomplications in process implementation and on human-centric operationsthat would otherwise be involved in developing and calibratingnon-standard processes, thereby reducing costs and the likelihood ofhuman error.

Overview of Stacked-Capacitor Architecture

Prior to describing various embodiments, the following section addressesan overview of an architecture for realization of improved high-voltagestacked floating-electrode capacitor structures. These may be needed forparticular areas within a CMOS device. For example, in standard CMOSdevices, a power supply voltage of 3.3 volts may be available. Most ofthe devices incorporated therein will be designed for this voltage butmay not operate reliably at higher voltages. However, there may be aneed in some areas of the IC for higher voltages such as 6 to 8 volts.For example, such may be desirable for programming certain types ofnonvolatile memories. The architecture of the stacked high-voltagecapacitor of FIG. 3 facilitates realization of such circuits, amongothers, and provides a framework within which various embodiments can bedeveloped.

FIG. 3 is an exemplary schematic diagram of an embodiment of a stackedhigh-voltage capacitor 300 that is compatible with formation viaconventional CMOS processes. In one embodiment, the stacked capacitor300 is formed from a first capacitor 305 and a second capacitor 310,each shown as a PMOS FET-like structure with source, drain and bodycontacts shorted together, although any MOS-like capacitor structuresusing floating gates, including NMOS FET-like structures, may beemployed.

The first capacitor 305 is shown as a PMOS FET-like structure withsource/drain and body electrodes coupled together to form a firstcapacitor electrode 315 providing electrical contact to a firstcapacitor plate of the first capacitor 305. The second capacitor 310 isshown as a PMOS FET-like structure with source/drain and body electrodescoupled together to form a second capacitor electrode 320 providingelectrical contact to a first capacitor plate of the second capacitor310. As used herein, the terms “FET-like” and/or “FET-type” are definedto mean a semiconductive body and including at least one contact node, adielectric disposed over at least a portion of a surface of thesemiconductive body and a conductive electrode disposed over at leastsome of the dielectric.

Floating electrodes 324 formed from conductive material, such as p-typeor n-type polysilicon and/or a metal such as aluminum, provide secondplates of the first and second capacitors 305, 310 and are electricallyconnected together at a common intercapacitor node 325. The floatingelectrodes 324 of the first and second capacitors 305, 310 may comprisethe same material or different materials coupled together via aconductive or ohmic interconnection. As used herein, the term “floatingelectrode” is defined to mean an electrode that is separated from aconductive or semiconductive body by an intervening dielectric havingthickness and other properties consistent with tunneling of chargecarriers between the floating electrode and the semiconductive body, incontrast to conventional floating-gate memory transistors, which employa floating gate together with a control gate, rather than a floatingelectrode sans control gate.

Conventional floating-gate memory transistors typically require acontrol gate stacked on a floating gate and separated therefrom by arelatively thick dielectric, and typically require a voltage directlyimpressed upon the control gate to initiate tunneling of charge carriersto the floating gate from a channel of the memory transistor (or viceversa) via a relatively thin dielectric separating the floating gatefrom the channel. In contrast, in the stacked-capacitorfloating-electrode devices described herein, one FET-like structure actsas a control gate for the other, and vice-versa, and tunneling can occurin either of the two FET-like structures (by reasons of symmetry)depending on the relative potentials of electrodes 315 and 320 andintercapacitor node 325.

The stacked capacitors described herein may be realized using technologysimilar to that of synapse transistors as described, for example, inU.S. Pat. Nos. 5,627,392; 5,825,063; 5,898,613; and 5,990,512 to Diorioet al., which patents are hereby incorporated herein by reference intheir entirety.

In the stacked capacitor 300 of FIG. 3, the floating electrodes 324 maybe charged or discharged via any of several mechanisms, such asFowler-Nordheim tunneling, Frenkel-Poole tunneling, direct tunneling, orinjection of electrons from the semiconductor body.

In one embodiment, with stacked capacitors as described herein, thesecharging and discharging mechanisms may allow the voltage on theintercapacitor node 325 to be self-adjusting. In other words, when ahigh voltage V_(CAP) is applied across the first 315 and second 320electrodes, there may initially be a disproportionate voltage dropacross one or the other of the capacitors 305 and 310, due to any of avariety of causes, such as differences in capacitance between them, orto the initial charge state on floating node 325.

In response to such an applied voltage V_(CAP), charge carriers maytunnel through one of the capacitor dielectrics such that, atequilibrium, any substantial imbalance in the voltage drop across thecapacitors 305, 310 is reduced, and ideally each capacitor 305, 310bears approximately one-half of the voltage V_(CAP), or V_(CAP)/2. As aresult, the stacked capacitor 300 is relatively robust, because neitherof the capacitors 305, 310 is required to maintain a disproportionateamount of the voltage V_(CAP) that is impressed on the capacitor 300once the capacitor 300 has equilibrated. Further, once the capacitor 300has equilibrated, the charge is stored on the floating gates 324 whilethe voltage V_(CAP) is present and for some time thereafter, with theresult that very little further charge tunneling/injection is required.

In other words, the intercapacitor node 325 self-adjusts to a voltagein-between that impressed on electrodes 315, 320 of the stackedcapacitor 300, and will tend to equilibrate to a voltage that is aboutor near one half of a working voltage. V_(CAP) impressed on the stackedcapacitor 300. A benefit associated with such a self-equilibratingstructure is that the breakdown voltage for the stacked capacitorstructure 300, assuming equal breakdown voltages and capacitances foreach of the capacitors 305, 310, is maximized when half of the totalvoltage V_(CAP) is applied across each of the capacitors 305 and 310.

In one embodiment, the voltage on the intercapacitor node 325 may beprogrammed with a charge-injection circuit, as is described in moredetail with respect to FIG. 9, infra, and in commonly assignedco-pending U.S. patent application Ser. No. 10/143,557, filed on May 9,2002, entitled “Metal Dielectric Semiconductor Floating Gate VariableCapacitor”, naming J. Hyde and Y. Ma as inventors, which is herebyincorporated herein by reference. Use of charge injection and adjustmentcircuits can permit the intercapacitor node voltage to be set to anappropriate value, even when the intercapacitor node value is not ableto self-adjust to such voltages in normal circuit operation.

Empirical studies show that forming the capacitors as PMOS FET-likestructures using p-type polysilicon floating electrodes can providemarkedly longer storage times for charge stored on floating gates than,for example, NMOS FET-like structures. Observations of differences incharge-storage times indicate that PMOS-type structures are capable ofstoring charge for up to 10⁴ times longer than comparable NMOS-typestructures. Exemplary storage times for PMOS-type structures with 7 nm(nanometer) thick oxides may be on the order of ten years, with 5 nmoxides providing storage times of from hours to days and with 3 nm oxidedielectrics providing storage times of seconds to minutes, however, itwill be realized that such storage times may be highlyimplementation-dependent.

Embodiments

FIG. 4 is an exemplary plan view of an embodiment 400 of the stackedhigh-voltage capacitor 300 of FIG. 3, and FIG. 5 is a side view, insection, taken along section lines V—V of FIG. 4, of an embodiment 500of the stacked high-voltage capacitor 300 of FIGS. 3 and 4. FIG. 4 showsa first capacitor 405 and a second capacitor 410. The first 405 andsecond 410 capacitors may be formed alike, as illustrated, or may beformed differently, as will be discussed in more detail below.

In one embodiment, the first capacitor 405 includes at least one firstnode 412, an optional second node 414, a conductor 415 contacting and/orinterconnecting the first node(s) 412 and the second node 414 andforming a first electrode 415 of the stacked capacitor 400 (analogous tothe first capacitor electrode 315 of FIG. 3). A conductor 420 forms asecond electrode 420 of the stacked capacitor 400 on the secondcapacitor 410 and establishes electrical contact with at least one node412 and may make electrical contact with the node 414 if such isincluded.

A floating electrode 424 forms one electrode of each of the first andsecond capacitors 405 and 410 and a portion 325 thereof also extendsbetween the first and second capacitors 405 and 410 (as represented bydashed lines in FIG. 5). This portion 325 also acts as theintercapacitor node 325 (as in FIG. 3). Each of the capacitors 405, 410is formed in a body of semiconductive material 435, 435′, with thebodies of semiconductive material 435, 435′ being electrically isolatedfrom each other, in contrast to conventional stacked MOS capacitors asin FIG. 2.

In one embodiment, the first capacitor 405 is formed from a PMOSFET-like structure having heavily doped source/drain nodes 412comprising p+ regions each disposed in a respective body of moderatelydoped n-type semiconductive material 435, 435′ and each such body 435,435′ optionally includes a heavily doped n+ region 414 as a bodycontact. Exemplary moderately doped levels for the bodies 435, 435′ arein a range of from 10¹⁶/cm³ to 10¹⁹/cm³, while exemplary heavily dopedlevels for the p+ 412 and n+ 414 nodes are in a range of from 10¹⁹/cm³to 5×10²⁰/cm³, although it will be appreciated that higher or lowerdoping levels may be employed.

In another embodiment, the p+ regions 412 are omitted and the n+ regions414 supply contact for the first and second electrodes 415, 420.Alternatively, the semiconductor bodies 435, 435′ may be p-type, witheither p+ contacts 412 and/or n+ contacts 414. The semiconductor bodies435, 435′ may be formed of the same or of different conductivity types(i.e. p-p, p-n, n-p, or n-n), and may be formed on semiconductive orinsulative substrates.

FIG. 5 is a side view, in section, taken along section lines V—V of FIG.4, of an embodiment 500 of the stacked high-voltage capacitor 300, 400of FIGS. 3 and 4, respectively. In the embodiment shown in FIG. 5, thecapacitors 505 and 510 employ interconnections 515 and 520,respectively, analogous to the first and second electrodes 415, 420 ofFIG. 4. A dielectric layer 530 is formed atop bodies of semiconductivematerial 535, 535′ formed at a top surface 540 (opposite a bottomsurface 540′) of a substrate 542.

In one embodiment, the bodies of n-type material 435, 435′ of FIG. 4comprise n-wells 535, 535′ (FIG. 5) disposed in a p-type semiconductoror insulating substrate 542. The nodes 412 and/or 414 and the n-wellsmay be doped as described above, while an exemplary p-type substratedoping is in a range of from about 10¹⁵/cm³ to about 10¹⁶/cm³, althoughit will be appreciated that higher or lower doping levels may beemployed. The dielectric layer 530 is disposed between the floatingelectrode 424 as well as the intercapacitor node 325 and portions of thetop surface 540 that include the n-wells 535, 535′. It will beappreciated that the bodies 535, 535′ may comprise p-wells formed in an-type or an insulating substrate 542. In one embodiment, one of thebodies 535, 535′ may be of an opposite conductivity type from another ofthe bodies 535, 535′ and the bodies may be isolated from one anotherand/or the substrate by p-n junctions or insulators.

The dielectric layer 530 may have a thickness in a range of from about50 nm to about 2 nm. In one embodiment, the dielectric layer 530 mayhave a thickness chosen in accordance with the storage times discussedabove with reference to the overview discussion associated with FIG. 3.

The dielectric layer 530 may be formed from silicon dioxide.Alternatively, the dielectric layer 530 may be formed from one or morematerials chosen from a group including nitrided oxide, nitride,oxide/nitride composition, titanium oxide, tantalum oxide, zirconiumoxide, hafnium oxide, lanthanum oxide (or any oxide of a lanthanide),titanium silicate, tantalum silicate, zirconium silicate, hafniumsilicate and lanthanum silicate (or any silicate of a lanthanide), anycombination of such dielectrics or from other insulating materials.

With any of these or other dielectric materials, thinner dielectriclayers 530 provide more capacitance per unit area but also provide lowerbreakdown voltages, and vice versa for thicker dielectric layers 530.These alternative dielectrics generally provide higher dielectricconstants and can therefore be utilized in thicker layers than silicondioxide.

The substrate 542 may be formed from semiconductive material, such asbulk monocrystalline silicon, or may include an insulator (e.g., siliconon insulator; silicon on sapphire; wafer bonded structures) or thedevices may be formed as TFTs (thin film transistors formed on aninsulative substrate) or as FinFETs (vertical FETs formed on ridges ofsemiconductive material with the gates disposed on one or both sides ofthe ridges), or as other lateral or vertical FET-like structures.

FIG. 6 is a side view, in section, analogous to that of FIG. 5, of anembodiment 600 of the stacked high-voltage capacitor of FIGS. 3 and 4.FIG. 6 shows a first capacitor structure 605 and a second capacitorstructure 610. The second capacitor structure 610 is formed in a well ofsemiconductive material 635, analogous to the wells 535 of FIG. 5, butformed within a second well of semiconductive material 650. The firstcapacitor structure 605 is formed in the well 650 but outside of theregion corresponding to the well 635. The well 650 is of an oppositeconductivity type to the well 635 and is also of an oppositeconductivity type to the substrate 642 (or substrate 642 may beinsulating).

In one embodiment, the well 635 is a p-doped region, the contact(s) 612are n+ contacts and the body contact 614 is a p+ contact. The capacitorstructure 610 is thus analogous to a NMOS FET or capacitor. The well 650may be a deep n-well, with the contact(s) 612′ being p+ contacts and theoptional body contact 614′ being an n+ contact. The substrate 642 isp-doped or insulating. In this embodiment, the parasitic p-n junctionsformed between the p-well 635 and the deep n-well 650, and between thedeep n-well 650 and the substrate 642, should both be reversed biasedfor appropriate operation.

In other words, a bias voltage applied to the deep n-well 650 should begreater than or at least equal to a bias voltage applied to thep-substrate 642, and a bias voltage applied to the deep n-well shouldalso be greater than or at least equal to the bias voltage applied tothe p-well 635. Accordingly, capacitor structure 605 should be “on top”in a voltage sense, that is, electrode 415 of capacitor structure 605should not be at a lower potential than electrode 420 of capacitorstructure 610.

Put another way, the capacitor structure 610 should be “on the bottom”in a voltage sense, and the body (p-well 635) of capacitor structure 610may be biased to a voltage below that of the substrate 642 (i.e., may bebiased to a voltage more negative than ground). These bias voltages arereadily provided using conventional circuits and techniques.

FIG. 7 is a side view, in section, analogous to that of FIG. 5, of anembodiment 700 of the stacked high-voltage capacitor of FIGS. 3 and 4.FIG. 7 shows a first capacitor structure 705 and a second capacitorstructure 710, each formed on an individual body of semiconductivematerial 535, 535′. In one embodiment, such comprises n-wells 535, 535′formed in a p-type substrate 542 together with a p+ or n+ contact 712 asdescribed above. However, it will be appreciated that other types ofsubstrates and semiconductor bodies may be employed.

When the semiconductor material 535, 535′ is n-type, n+ contacts 712 maybe appropriate. When the semiconductor material 535, 535′ is p-type, p+contacts 712 may be appropriate. These semiconductor type and contacttype may be the same or different for the capacitors 705 and 710.

In any case, the contact 712 of the capacitor 705 is coupled to theterminal 415 and the contact 712 of the capacitor 710 is coupled to theterminal 420. Adding additional optional n+ and/or p+ contacts to thestructure 700 causes the structure 700 to more nearly resemble thestructure 500 of FIG. 5. Similarly, removal of portions of the structure500 of FIG. 5 causes such to more nearly resemble the structure 700 ofFIG. 7.

It will be apparent to those of skill in the art that the variousembodiments described herein may be combined in numerous ways to realizestacked capacitors. For example, NMOS FET-type and PMOS FET-type, NMOSor PMOS-type capacitors, or accumulation mode and inversion modestructures (or admixtures thereof) may be combined in series or inparallel to provide a desired result.

Exemplary Self-Adjustment of the Floating-Gate Charge

FIGS. 8A and 8B are band diagrams reflecting different bias andtunneling conditions relative to the embodiments of FIGS. 3–7. In FIGS.8A and 8B, the ordinate corresponds to increasing negative potentialwhile the abscissa corresponds to position. For example, potentials A-Drepresent increasing positive potentials, such as −3.1, 0, 3.1 and 10volts, respectively, while position α corresponds to a portion of thehigh-voltage capacitor (e.g., 305, FIG. 3) beneath the gate dielectricand floating gate 324, such as the channel of a FET forming ahigh-voltage capacitor or the body of the high-voltage capacitor, andposition β corresponds to the floating-gate electrode (e.g., 324, FIG.3). The region between α and β corresponds to a dielectric (e.g. 530,FIG. 5).

FIG. 8A depicts a bias scenario where relatively little tunnelingoccurs, and illustrates portions 800, 802, 805 and 810 of a curvedescribing electrical potential. The portion 800 of the potential curve,corresponding to the conduction band of the body or channel, is atpotential B; the portion 805 corresponds to the conduction band of thedielectric; the portions 802 correspond to the forbidden gap of thedielectric; while a portion 810, corresponding to the floatingelectrode, is at potential C. The portion between 800 and 805,corresponding to positions B and A on the ordinate, respectively,represents a potential barrier to the conduction of charge carriers,such as electrons, from the channel or body 800 to the floatingelectrode 810. The portion between 800 and 810, corresponding to thepositions α and β, on the abscissa, represents a potential barrier totunneling of charge carriers, such as electrons, from the channel orbody 800 to the floating electrode 810.

FIG. 8B depicts a bias scenario favorable to increased tunneling ofcharge carriers. A portion 820 of the potential curve, analogous to theportion 800, is at potential B, while a portion 830 is at potential D.As a result, the portion 825 has a much steeper slope than thecorresponding portion 805 of FIG. 8A. In turn, the thinned extent of theforbidden gap comprising the leftmost portion 802 (position α) and thecorresponding portion of the curve 825 (position γ) facilitatestunneling of charge carriers through the forbidden gap to a location andpotential such as position γ of the curve 825 at the potential C (thetunneling typically represented as shown by arrow 835), followed byacceleration towards the portion 830.

Electron conduction (via tunneling) from channel or body 820 to floatinggate 830 decreases the electrostatic potential of floating gate 830.Such causes the potential on the floating gate 324 to adjust to a valuesuch that the overall shape of the potential curve becomes sufficientlylike the shape of the potential curve of FIG. 8A that tunneling is lessprobable. In practice, the potential may continue to self-adjust butprogressively (e.g., exponentially) more slowly. This self-adjustmentprocess may take place in either capacitor 305 or 310 until thepotential of intercapacitor node 325 self-adjusts to levels wherefurther tunneling in capacitor 305 or 310 is less and less likely. Whilethe example of FIGS. 8A and 8B depicts a scenario corresponding toFowler-Nordheim tunneling, it will be apparent to those of skill in theart that similar self-adjustment can take place with other mechanismssuch as Frenkel-Poole tunneling or direct tunneling. It will also beapparent that the self-adjustment process can take place when eithercapacitor 305 has a larger voltage across it than capacitor 310, or viceversa, the self-adjustment process acting to adjust the charge onintercapacitor node 325 and thereby decrease the voltage acrosscapacitor 305 and increase that across capacitor 310, or vice versa.Finally, it will be apparent, by reasons of symmetry, that chargecarriers such as electrons can tunnel in the opposite direction, fromfloating gate to channel or body, rather than from channel or body tofloating gate, under appropriate bias conditions.

It will be apparent to those of ordinary skill in the art that chargeneed not transfer all the way from channel or body to floating gate (orvice versa), but, rather, can equalize the voltage across capacitors305, 310 by accumulating in dielectric layer 530. One example is SONOSdevices (silicon-oxide-nitride-oxide-silicon structures) in whichinjected charge is trapped in an insulative nitride layer sandwichedbetween oxide layers.

Exemplary Charge Injection/Removal Circuit

FIG. 9 is a schematic diagram of a high-voltage capacitor circuit 900incorporating a charge injector 905 and a charge drain 910. In oneembodiment, stacked capacitor 915 includes a common node 925 comprisinga floating gate, as described above, and is coupled to the chargeinjector 905 and to the charge drain 910.

In one embodiment, the devices forming the charge injector 905, thecharge drain 910 and the stacked capacitor 915 all share a commonfloating gate 925, which may be formed from a metal or from p- or n-typepolysilicon. In one embodiment, the structures making up the capacitorsin the stacked capacitor 915 each have a relatively large area (e.g., 9μm×9 μm) while the structures forming the charge injector 905 and thecharge drain 910 are substantially smaller (e.g., 0.24 μm×0.60 μm).

In one embodiment, the structures making up the capacitors in thestacked capacitor 915 have approximately the same area and approximatelythe same capacitance. In one embodiment, the structures forming thecapacitors in the stacked capacitor 915 are substantially larger (e.g.,500×) than those forming either the charge injector 905 or the chargedrain 910. Such can obviate effects on the capacitance of the stackedcapacitor 915 arising from parasitic capacitance effects resulting fromthe charge injector 905 and the charge drain 910.

In one embodiment, the charge injector 905 may be formed as a FET havingone electrode coupled to a voltage V_(INJ), with the body and anotherelectrode coupled to a voltage V_(BIAS). A relatively largedrain-to-source voltage, (e.g., |V_(BIAS)−V_(INJ)|) causes electrons(e.g., channel electrons in the case of an nFET, or secondary electronsgenerated by impact ionization in the case of a pFET) to inject onto thecommon electrode 925. In one embodiment, the FET is a p-channel MOSFET,V_(BIAS) is about 3.3V, and V_(INJ) is about −2.5V.

In one embodiment, the charge drain 910 may be formed as a shortedPFET-like structure in an n-well, i.e., much like structures describedabove with reference to 405 or 410 in FIG. 4. Application of arelatively large potential to the charge drain 910 allows chargecarriers to tunnel from the floating electrode 925, through dielectric930, to the charge drain 910. For example, a positive voltage V_(TUN)may be applied to cause electrons to tunnel from the floating electrode925 and thus increase the amount of charge stored in the stackedcapacitor circuit 915.

Other charge injection and drain circuits analogous to the chargeinjector 905 and charge drain 910 of FIG. 9 may also be used to adjustthe charge stored on floating gate 925 of stacked capacitor 915.Alternatively, two charge injectors (of opposing polarity) may be usedin place of an injector a drain, or two charge drains (of opposingpolarity) may be used in place of an injector a drain. It will beappreciated that the stacked capacitor 915 may need to be modified toprovide appropriate biases in order to employ these alternativeinjection and drain circuits.

Exemplary Circuit Application

FIG. 10 is a schematic diagram of an exemplary voltage-multipliercircuit 1000 advantageously employing the capacitor structures of any ofFIGS. 3–7. The voltage multiplier circuit 1000 acts to provide a highvoltage V_(OUT) such as about eight or more volts from a supply voltageV_(DD) such as 3.3 volts and operates in response to a clock signalCLOCK.

The voltage multiplier circuit 1000 includes a first diode 1005 havingan anode coupled to the supply voltage V_(DD) and having a cathode. Afirst capacitor 1010, which may be formed from a FET (as illustrated)has power electrodes coupled to the cathode of the first diode and to ananode of a second diode 1015, and has a control electrode. The firstcapacitor 1010 need not be a high-voltage capacitor.

A cathode of the second diode 1015 is coupled to an electrode of a firststacked high-voltage capacitor 1020 formed from two capacitors 1022 and1024. A third diode 1030 has an anode coupled to both the cathode of thesecond diode and to the first stacked high-voltage capacitor 1020. Acathode of the third diode 1030 is coupled to a second stackedhigh-voltage capacitor 1040 formed from two capacitors 1042 and 1044.

The clock signal CLOCK is buffered through a first inverter 1050 andcauses the gate or control electrode of the first capacitor 1010 toalternately be grounded or be coupled to V_(DD). When the gate isgrounded, the first capacitor 1010 is charged to approximately V_(DD)(assuming that the forward voltage drop across diode 1050 is small).When the gate or lower electrode is raised to V_(DD) in response to achange in the clock signal, the FET body or upper capacitor electrode isthen raised in voltage to approximately 2×V_(DD), and cannot leak thecharge Q back to the V_(DD) supply because of the diode 1005.

At the same time, the clock signal is buffered through a second inverter1055 such that the stacked capacitor 1020 comprising capacitors 1022 and1024 has its lower electrode grounded. As a result, charge is dumpedinto the stacked capacitor 1020 from the first capacitor 1010, raisingthe voltage on the stacked capacitor 1020 to approximately 2×V_(DD).

On a next clock signal transition, the lower electrode of the capacitor1024 is raised to V_(DD), with the result that the upper electrode ofthe stacked capacitor 1020 (the source/drain/well contacts of thecapacitor 1022) is raised to a voltage approximately 3×V_(DD) and dumpscharge through diode 1030 into the stacked capacitor 1040 formed bycapacitors 1042 and 1044. After several clock cycles, stacked capacitor1040 is charged to approximately 3×V_(DD). The associated intercapacitornodes of stacked capacitors 1020 and 1040 likewise charge to valuesin-between the respective working voltages and ground, via theself-equilibration process described above.

As a result, the voltage V_(OUT) is charged to approximately 3×V_(DD),minus the forward voltage drops across the diodes 1005, 1015, and 1030,and other factors. Because the voltages sustained by the stackedcapacitors 1020 and 1040 are greater than V_(DD), these capacitors needto be high-voltage capacitors in order for the circuit 1000 to workreliably. The stacked capacitor structures described herein facilitaterealization of high-voltage capacitors and may do so while beingconsistent with conventional CMOS processes.

CONCLUSION

The concepts described herein provide significant advantages compared toprior art high-voltage CMOS-compatible capacitors. The stackedfloating-gate high-voltage capacitor structures do not require a controlgate or the circuitry ordinarily associated therewith and do not requirethe biasing circuitry typically employed in conventional stackedcapacitor structures. As a result, these floating-gate high-voltagecapacitor structures require fewer processing operations to fabricateand thus are relatively robust in manufacturing. The capacitorsdescribed herein may incorporate an intercapacitor node that isself-adjusting with respect to a working voltage impressed across thestacked floating-gate high-voltage capacitor.

Additionally, the stacked floating-gate high-voltage capacitor storescharge from an initial adjustment of the intercapacitor floating nodefor relatively long periods of time, even in the absence of externallyapplied electrical signals. The stacked high-voltage capacitor may beemployed in a configuration that does not require any electricalconnection to the intercapacitor node other than that of the floatinggates themselves, simplifying fabrication and operation of the resultinghigh-voltage capacitor structure.

Although the disclosed concepts have been described in language specificto structural features and/or methodological acts, it is to beunderstood that the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described. Rather,the specific features and acts are disclosed as exemplary forms ofimplementing the claimed subject matter.

1. A method comprising: in a stacked capacitor having a first capacitorhaving a first plate comprising a first semiconductive body and a secondplate comprising a first floating electrode, and a second capacitorhaving a first plate comprising a second semiconductive body and asecond plate comprising a second floating electrode, the first andsecond semiconductive bodies being electrically isolated from eachother, the first and second floating electrodes being connected togetherat an intercapacitor node, coupling a first potential to the firstelectrode; coupling a second potential to the second electrode, adifference between the first and second potentials comprising a workingvoltage of the stacked capacitor; and charging the intercapacitor nodeto a floating voltage whose value is less than the working voltage. 2.The method of claim 1, wherein the first and second floating electrodesare only electrically connected to each other and to the intercapacitornode.
 3. The method of claim 1, wherein additional circuit elements areelectrically connected to the intercapacitor node, each of theadditional circuit elements chosen from a group consisting of: acapacitor, a resistor, a diode, a field-effect transistor, a NFET, aPFET, a MOSCAP, a bipolar-junction transistor, a metal-semiconductorfield-effect transistor, a FinFET, a vertical-gate transistor and aninsulated-gate transistor.
 4. The method of claim 1, wherein the firstand second floating electrodes comprise a material chosen from a groupconsisting of: polysilicon, semiconductor material and a metal.
 5. Themethod of claim 1, wherein the first and second semiconductive bodiescomprise doped silicon.
 6. The method of claim 1, further comprisingself-adjusting the intercapacitor node to a value less than a workingvoltage impressed on the stacked capacitor.
 7. The method of claim 1,further comprising storing a voltage at the intercapacitor node whenelectrical power is removed from the stacked capacitor.
 8. The method ofclaim 1, further comprising: adjusting a floating voltage of first andsecond floating electrodes to a value less than a working voltage of thestacked capacitor.
 9. The method of claim 1, wherein the first andsecond capacitors are configured to be further responsive to actionsincluding storing the floating voltage on the floating electrodefollowing removal of the first and second potentials from the stackedcapacitor.
 10. The method of claim 1, wherein the first and secondcapacitors are configured to be responsive to acts including tunnelingcharge carriers through a dielectric, wherein the dielectric includes afirst portion disposed between the first semiconductive body and acorresponding portion of the floating electrode and the dielectricincludes a second portion disposed between the second semiconductivebody and a corresponding portion of the floating electrode.
 11. Themethod of claim 1, wherein coupling the first potential to the firstelectrode and coupling the second potential to the second electrodecomprise coupling the first and second potentials to first and secondelectrodes of the stacked capacitor, and wherein the stacked capacitorcomprises a plurality of PFETs, NFETs or MOSCAPs, or any combinationthereof coupled to the common intercapacitor node.